1. Field of the Invention
The present invention relates to a semiconductor device using a thin-film semiconductor. More particularly, the present invention relates to a structure of a gate electrode in an insulated gate type transistor.
2. Description of the Related Art
As a semiconductor device using a thin-film semiconductor, attention is attached to a thin-film transistor (TFT). Especially, in recent years, a TFT capable of performing high speed operation by using a crystalline silicon film (for example, polysilicon film), has been put into a practical use.
Although a thin-film transistor using a crystalline silicon film as an active layer has a high mobility (field effect mobility), it has such a defect that an off-state current (current flowing when the TFT is in an off-state) is large. Further, the thin-film transistor has a problem that when the mobility becomes high, a withstand voltage becomes low so that deterioration becomes noticeable.
As means for solving such problems, there is known a technique disclosed in Japanese Examined Patent Publication No. Hei 5-44195. According to this technique, a thin-film transistor is made equivalently to have such a structure (also called as a multi-gate structure) that a plurality of thin-film transistors are connected in series to each other, so that an applied voltage is distributed to each of the plurality of thin-film transistors.
FIG. 4 is a structural view showing an active layer and a gate electrode of a thin-film transistor manufactured by using the technique disclosed in the above publication. In FIG. 4, reference numeral 401 denotes a source region, and 402 denotes a drain region. Gate electrodes 403 to 406 are formed above the active layer through a gate insulating film (not shown). At this time, the gate electrodes 403 to 406 are formed integrally so that they are connected electrically.
Channel forming regions 407 to 410 are formed just under the gate electrodes 403 to 406 into shapes corresponding to those of the gate electrodes 403 to 406. It is characterized in that the structure is substantially composed of a plurality of thin-film transistors commonly connected in series.
However, according to experiments carried out by the present inventors by using the TFTs having the structure as shown in FIG. 4, it has been found that the thin-film transistor closest to the drain region 402 deteriorates most intensely. Also, it has been found that when a high voltage is applied between the source and drain, breakdown or deterioration proceeds sequentially from a transistor at the side close to the drain region.
According to another experiment, it has been found that in a TFT constituted by an active layer with a wide channel width, the vicinity of a center of an active layer (vicinity of the center in the channel width direction) deteriorates most intensely.